The Hers-Vif (, "Live Hers", as opposed to the slower flowing Hers-Mort, "Dead Hers"), also named Grand Hers or simply Hers, is a 135 km long river in southern France, right tributary of the Ariège. The Hers-Vif rises at an elevation of about 1500 m near the Chioula Pass of the Pyrenees, approximately 6 km north of Ax-les-Thermes. It is the major tributary of the Ariège into whose right bank it flows 2 km upstream from Cintegabelle in the Haute-Garonne. It flows some 30 km through the Pyrenees, descending 1100 m to the village of Peyrat, where it reaches a piedmont plain. Its valley widens as it traverses the plain, reaching the medieval city of Mirepoix, which marks the start of its lower valley. Several rivers flow into it: Departments and towns along its course are:

Floods

The Hers is probably known as vif (intense or rapid in this context) because of its sometimes spectacular floods – that of 16 June 1289 having entirely destroyed Mirepoix. More recently, there have been: